Process for the manufacture of stainless steel



United States Patent 3,396,014 PROCESS FOR THE MANUFACTURE OF STAINLESS STEEL Naaman H. Keyser, Par-ma, Ohio, assiguor to Interlake SEteeIlK Corporation, Chicago, 111., a corporation of New No Drawing. Filed June 3, 1965, Ser. No. 461,189

21 Claims. (Cl. 75130.5)

ABSTRACT OF THE DISCLOSURE A process for producing stainless steel comprising providing a ferrous basis melt having a chromium content of 14%, subjecting the basis melt to an oxygene blow at a temperature range of approximately 2900 F. to 3000 F. to reduce the carbon content to produce a decarburized basis melt having a silicon content of at least 0.25%, adding low carbon content chromium source material to the basis melt, continuing the oxidizing blow, and maintaining the silicon level during the introduction of the low carbon content chromium source material and the continued oxidizing blow.

This invention relates to a process for the production of stainless steel containing chromium and more particularly relates to a process for making stainless steel in a basic oxygene furnace.

Previous processes for producing chromium bearing stainless steel in a basic oxygen furnace results in a substantial oxidation of the chromium which passes into the slag. Such prior processes produce somewhat undesirable results in that the chromium oxide when it reaches certain levels in the slag make the latter viscous and unmanageable from the operating standpoint, and furthermore, the chromium oxide is too valuable to discard and the slag must therefore be reprocessed to recover the chromium metal.

In the co-pending US. patent application of Thaddeus F. Bell, et a1. Ser. No. 134,033, filed Aug. 24, 1961, now Patent No. 3,198,624, there is a disclosed a process for the production of stainless steel in which the oxidation of the chromium is lessened by conducting the oxidation treatment at elevated temperature levels. Such elevated temperature levels require an expenditure of more fuel, and thus is not as economically desirable as may be necessary, and also destruction and erosion of the refractory lining of the furnace increases due to the elevated temperature, resulting in increased costs of maintenance on the equipment used in the process.

The present invention provides a novel process for the production of chromium bearing stainless steel, and one which involves the operating of a basic oxygen furnace at a somewhat lower temperature than has heretofore been possible for obtaining a good recovery of chromium in the melt, while at the same time achieving even greater percentage recoveries and better economy of the chromium, and without undesirable results concerning the refractory lining of the furnace. Thus, the present method diminishes the amount of chromium which is oxidized from the melt during the oxidizing blow, resulting in an improvement in the cost of the final stainless steel product.

Accordingly, it is an object of the present invention to provide a novel process for the production of stainless steel containing chromium as a major element.

Another object of the invention is to provide a novel process for the production of stainless steel containing chromium wherein a basis melt embodies a relatively low chromium centent, comprising subjecting the basis melt to an oxidizing blow to reduce the carbon content thereof, and then adding a relatively low carbon chromium bearing material to the decarburized basis melt and continuing the oxidizing blow to produce a stainless steel having a higher percentage of chromium therein from the percentage of chromium that existed in the original basis melt.

A still further object of the invention is to provide a process of the above-mentioned type which may be carried on at temperatures which will not have great adverse eflects upon the refractory lining of the furnace in which the process is being carried on.

A still further object of the invention is to provide a process of the above-mentioned type wherein the silicon content of the melt during the addition of the low carbon chromium bearing material and the continuation of the oxidizing blow, is maintained at least at 0.25%, for re stricting oxidation of the chromium.

Other objects and advantages of the invention will be apparent from the following description:

In the usual stainless steel making operations, the chromium is prone to oxidize during the reduction of the carbon in the melt. In an attempt to reduce the oxidization of the chromium, the temperature of the melt has been increased to temperatures of 3100 F. and greater, to cause a reduction of the oxidation of the chromium. However, such high temperatures have adverse effects upon the refractory linings of the vessel in which the basis melt is contained. Moreover, even with the use of increased temperatures a substantial portion of the chromium may still be oxidized during the blowing operation.

It has been determined that there is a relationship between the recovery of the chromium in the final product and the initial level of the chromium in the basis melt when the oxidizing blow commences. The higher the level of chromium when the oxidizing blow starts, the more chromium percentage wise is oxidized and transferred to slag by the time decarburization has proceeded to the desired extent.

At temperature levels low enough to result in reasonably low refractory consumption, or in other words, at temperatures between approximately 2900 F. to 3000 F., the recovery of the chromium falls off sharply when the chromium level in the basis melt exceeds approximately 8 to 12%. This drop is so sharp that when initial levels of chromium in the basis melt are in the range of approximately 15 to 20%, up to about half or more of the chromium may be oxidized and transferred to the slag. Initial levels of chromium of 8 to 14% or lower, result in much lower oxidation of the chromium at reasonable temperature levels, but the resultant product does not generally contain enough chromium for many of the popular grades of stainless steel.

The present process takes advantage of the greater economy of chromium oxidation which can be achieved by decarburizing relatively low chromium levels, and then the chromium level is built up with a relatively low carbon chromium containing material to produce the desired chromium level in the final product. During the addition of the relatively low carbon chromium containing materials to the decarburized basis melt, it has been found to be necessary that the silicon level in the decarburized basis melt be maintained at least at 0.25% or greater, to effectively restrict oxidation of the chromium in the melt during the subsequent gaseous blow.

The following is a specific example of the utilization of the novel process in the production of chrome bearing stainless steel.

There was produced in a furnace a melt consisting of 7030 lbs. of molten iron containing approximately 0.92% silicon, approximately 4.0% carbon, the rest being substantially all iron and the usual impurities. This molten metal was charged into a basic lined (e.g. magnesia lined) oxygen refining vessel and there was added thereto 500 lbs. of lime and approximately 170 lbs. of ferrosilicon containing approximately 73.1% silicon and approximately 0.15% carbon. A gaseous blow of 6000 cu. ft. of oxygen was then applied to the melt by means of a water cooled lance to the upper surface of the melt, to oxidize the silicon and phosphorus, which produced a slag which was then removed. Then to the molten melt, there was added 40 lbs. of ferrosilicon containing approximately 73.1% silicon and approximately 0.15% carbon, together With 930 lbs. of high carbon ferrochromium containing between approximately 1% to 2% silicon, between approximately 4.5% to 6% carbon and between approximately 67% to 70% chromium. There was also added approximately 210 lbs. of lime.

5000 cu. ft. of oxygen was then blown into such basis melt to decarburize the metal. To such decarburized basis melt there was then added 100 lbs. of ferrosilicon of approximately 73.l% silicon and approximately 0.15% carbon, 900 lbs. of low carbon ferrochromium comprising approximately 0.3% to 1% silicon, approximately 0.03% to 2% carbon, and approximately 68% to 71% chromium, together with 310 lbs. of lime. An additional 4800 cujft. of oxygen was blown during and following the addition of low carbon ferrochromium.

From such process there was recovered 8800 lbs. of stainless steel metal with an analysis of approximately 0.20% carbon, 0.25% silicon and 10.33% chromium.

While the low carbon chromium containing material in the above example was ferrochromium, other low carbonhigh chromium bearing materials, such as ferrochromium silicon may also be used.

The chromium recovered in the finalized product was approximately 76% of the metallic chromium that was charged into the melt, and the silicon content of the decarbonized basis melt was at a level of at least 0.25% during the introduction of the low carbon ferrochromium and the subsequent oxygen blow. Using prior art processes usually results in the recovery in the final product of only approximately 51% of the chromium charged.

Other desirable alloys may be introduced into the oxygen refining vessel during the introduction of the low carbon chrome bearing material or at the conclusion of the oxidizing blow. Those elements which have a very high atfinity for oxygen, such as aluminum, calcium, titanium, zirconium, etc. can be added after the oxidizing blow and in the usual fashion. Other elements, such as nickel, molybdenum, tungsten, tantalum, and boron may also be added to the melt in the conventional manner.

The oxygen utilized in the above described process may be of the industrial grade and may be applied at the rate of approximately 100 to 400 cu. ft. per minute. The reaction is exothermic and thus increases the temperature of the melt.

It has been determined that the following ranges generally provide for practicing the invention as above discussed in the production of stainless steel.

A basis melt comprising approximately 0.1% to 6.0% car-hon, approximately 4% to a maximum of 14% chromium, and approximately 0.4% to 4.0% silicon, the balance being essentially iron with possibly some nickel and the usual impurities.

A reduction of the carbon content of the basis melt by an oxidizing blow, to a level of between approximately 0.04% to 1.0% and at a temperature of approximately 2900 F. to 3000 F.

The ratio by weight of the basis alloy after decarburization to the subsequently added low carbon chromium bearing material being in the range of approximately :1 to 3:1.

The provision of a silicon content of the decarburized basis melt of at least 025% before addition of the low carbon chromium, and maintenance of said minimum silicon level during the subsequent oxidizing blow.

The weight of added lime being in a range sutficient to keep the slag basic in nature and fluid in accordance with good steel making practice.

The amount of oxygen applied by blowing being adjusted to supply suflicient heat to melt all of the solid additions and raise the temperature of the melt to the desired temperature for tapping.

Such a process as above described results in a stainless steel alloy comprised of between approximately 10% to 20% chromium, between approximately 0.04% to 1.0% carbon, and between approximately 0.25% to 2.0% silicon.

From the foregoing, it will be seen that the invention provides a novel process for the production of chromium bearing stainless steel, and one that utilizes an oxidizing blow in the decarburization of the steel, and one that makes the most effective use of the chromium units in the basis melt, resulting in a greater recovery of the chromium metal in the finalized product.

The terms and expressions which have been used are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of any of the features described, or portions thereof, and it is recognized that various modifications are possible within the scope of the invention claimed.

I claim:

1. The process for the production of stainless steel comprising preparing a molten chromium bearing basis melt containing between 0.1% to 6.0% carbon, approximately 4% to 14% chromium, and between approximately 0.4% and 4.0% silicon, and with the balance comprising essentially iron and the usual impurities, blowing said basis melt with oxygen in a vessel until the carbon content is reduced to a level of between approximately 0.04% to 1.0% to produce a decarburized basis melt, adding low carbon chromium containing material to the decarburized basis melt and continuing the oxidizing blow to produce an alloy comprised of between 10% to 20% chromium, 0.04% to 1% carbon and at least approximately 0.25% silicon, including providing said decarburized basis melt with at least 0.25% silicon, maintaining said silicon level during the introduction of said low carbon chromium containing material and the subsequent oxidizing blow, and the maximum temperature of said basis melt being within the range of approximately 2900 F. to 3000 F. during application of the oxidizing blow.

2. A process in accordance with claim 1 wherein said low carbon chromium containing material comprises low carbon ferrochromium of approximately 0.3% to 1% silicon, approximately 0.03% to 2.0% carbon, and approximately 68% to 71% chromium.

3. A process in accordance with claim 1 including adding of lime to said decarburized basis melt with said low carbon chromium containing material.

4. A process in accordance with claim 2 wherein the ratio by weight of said decarburized basis melt to the low carbon ferrochromium is approximately 10:1 to 3:1.

5. A process for the production of chromium bearing stainless steel including preparing in a furnace a molten alloy of ferrous material comprising between approximately 0.1% to 6.0% carbon, between approximately 4% to 14% chromium, and between approximately 0.4% to 4.0 silicon and the balance consisting essentially of iron and the usual impurities, blowing the molten alloy with an oxygen blow to decarburize the melt and reduce the carbon to between approximately 0.04% to 1.0%, adding low carbon ferrochromium and ferrosilicon to the decarburized melt, and then continuing the oxygen treatment to produce a molten alloy comprising between approximately 10% to 20% chromium, between approximately 0.04% to 1.0% carbon, and between approximately 0.25% to 2.0% silicon, including providing a silicon level of at least 0.25% to said decarburized melt and maintaining said silicon level during the introduction of said low carbon ferrochromium and the subsequent oxygen treatment, and the melt being maintained at a temperature of approximately 2900" F. to 3000 F. during the introduction of the low carbon ferrochromium.

6. A process in accordance with claim 5 wherein said ferrosilicon comprises between approximately 73% to 78% silicon and approximately 0.15% carbon.

7. A process in accordance with claim 5 wherein the ratio by weight of said decarburized melt to said added ferrochromium and ferrosilicon is within the range of approximately :1 to 3:1.

8. A process in accordance with claim 5 wherein a slag is formed upon application of the first mentioned oxygen blow, and including separating the slag from the melt prior to adding the low carbon ferrochromium.

9. A process for producing stainless steel, the steps comprising providing a ferrous basis melt having a chromium content of 4% to 14%, subjecting said basis melt to an oxygen blow at a temperature within the range of approximately 2900" F. to 3000 F. to reduce the carbon content to produce a decarburized basis melt having a silicon content of at least 0.25%, adding low carbon content chromium source material to said basis melt, continuing the oxidizing blow, and maintaining said silicon level during the introduction of said low carbon content chromium source material and the continued oxidizing blow to produce stainless steel having a higher percentage of chromium therein than that in the initial basis melt with the temperature of the melt being maintained at approximately 2900 F. to 3000 F. during the oxygen blow.

10. A process in accordance with claim 9 wherein the ratio of said basis melt after decarburization to said low carbon content chromium source material is in the range of 10:1 to 3:1.

11. A process in accordance with claim 9 wherein said process is carried out in a basic lined reaction vessel.

12. A process in accordance with claim 9 wherein lime is added during saidp rocess in sutficient weight to keep the slag from said basis melt basic and fluid.

13. A process in accordance with claim 9 wherein said oxygen blow is applied at a rate within the range of 100 to 400 cubic feet per minute.

14. A process in accordance with claim 9 wherein said basis melt initially comprises a carbon content within the approximate range of 0.1% to 6.0%, a chromium content within the approximate range of 4% to 14%, a silicon content within the approximate range of 0.4% to 4.0% and the balance being substantially all iron.

15. A process in accordance with claim 14 wherein the carbon content of said decarburized basis melt is within the range of approximately 0.04 to 1.0%.

16. A process in accordance with claim 15 wherein the carbon content is reduced to said range by the oxygen blow before said low carbon content source material is added to said basis melt.

17. A process in accordance with claim 9 wherein said low carbon chromium source material is selected from the group consisting of ferrochromium and ferrochromium silicon.

18. A process in accordance with claim 9 wherein said silicon content is maintained by the addition of a material selected from the group consisting of ferrosilicon and ferrochromium containing silicon.

19. A process in accordance with claim 9 wherein said low carbon chromium source material comprises low carbon ferrochromium with a silicon content of approximately 0.3% to 1%, a carbon content of approximately 0.03% to 2.0% and a chromium content of approximately 68% to 71%.

20. A process in accordance with claim 19 wherein said process includes addition of lime to said decarburized basis melt with said low carbon chromium source material.

21. A process in accordance with claim 20 wherein said process includes the addition of ferrosilicon including a silicon content of approximately 73.1% and a carbon content of approximately 0.15% to maintain said silicon content at least at 0.25

References Cited UNITED STATES PATENTS 2,226,967 12/1940 Chelius 7560 X 2,557,458 6/1951 Ogan 75-130.5 X 2,704,247 3/1955 Connor 75-130.5 X 3,003,865 10/1961 Bridges 75-1305 X 3,172,758 3/1965 Jandras 75-130.5 3,218,157 11/1965 Dobrowsky et a1. 75130.5 X

OTHER REFERENCES Electric Furnace Proceedings--1963; AIMl; vol. 21; pp. 17, 25, 26.

HYLAND BIZOT, Primary Examiner.

H. W. TARRING, Assistant Examiner. 

